1. Field of the Invention
The present invention relates to a drum brake that is attached to a vehicle, for example, an automobile.
2. Description of the Prior Art
One type of known drum brake is designed not only to serve as an ordinary brake that brakes a running vehicle but also to function as a parking brake.
One example of this type of drum brake comprises a pair of brake shoes which are provided such that when expanded, they are forced against a brake drum to generate a braking force, a strut which extends into notches formed in the respective webs of the brake shoes so as to extend therebetween, a parking lever which is pivotally supported on either one of the brake shoes such that when rotated, it engages the strut to expand the brake shoes, and a coil spring which is loaded between the strut and the notch formed in the web of the other brake shoe to bias the strut toward the first brake shoe.
In this drum brake, when the parking lever is pivoted, it causes the strut to press the second brake shoe against the brake drum, and the counterforce resulting from this action causes the first brake shoe to be pressed against the brake drum, thereby functioning as a parking brake. The coil spring, which biases the strut in one direction, prevents a backlash in the longitudinal direction of the strut when subjected to vibration.
The above-described prior art suffers, however, from the problems stated below.
Although the coil spring can prevent longitudinal backlash of the strut by biasing it toward the first brake shoe, it cannot prevent backlash of the strut in a direction which intersects the longitudinal direction. For this reason, if the strut moves in the notch formed in the web due to backlash, the coil spring, which is interposed between the strut and the web, is damaged by the edges of the web defining the notch (i.e., the corners where the obverse or reverse surface of the web intersects the inner peripheral surface of the web defining the notch, which are denoted by A in FIG. 3), thus causing rusting of the coil spring and a lowering of the biasing force. In the worst case, the coil spring is broken.